Engine reving high after driving

I just changed my O2 sensor in my 91 olds calais 2.3 quad 4 and it starts fine and then after a short ride when i put in the park (or
neutral) the engine revs high. But when i put it in neutral and let it roll to approx. 5 mph the reving goes back to normal but when i stop it revs high again. Any help is appriciated. Thanks in advance.
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